The present invention related generally to the dicing of semiconductor devices and more specifically to a saw blade assembly for separating devices on an unsupported substrate.
In the semiconductor industry separation of a processed wafer into its individual chips has evolved from techniques such as scribing and breaking, or laser dicing into an automated process using a dicing saw with a rotating circular blade. The process and equipment allow automation, accuracy, cleanliness, and versatility in selection of depth and width of the cut, and has become the technique employed throughout the industry.
Typically a semiconductor wafer is supported on a flat, rigid vacuum chuck and a high speed rotating blade with embedded hard, abrasive particles is programmed to saw the streets between the chips in first the xe2x80x9cxxe2x80x9d direction, and then the substrate is rotated ninety degrees to saw in the transverse direction. As complexity of the devices has increased expensive and dissimilar materials are often combined to produce multiple layers in the devices which adds to the difficulty of dicing accurately.
The abrasive material of the saw blade is most frequently diamond particles embedded in a softer material matrix to form blades. The exposed portion of the dicing blades are thin, in the range of 0.0005 to 0.002 inches thick which enables cutting to precise dimensions with smooth edges defined on the diced object, while minimizing the amount of costly semiconductor substrate abraded during the process. The exposed area of the blade is sufficiently large enough to saw completely through the object, but is kept as small as possible in order to minimize breakage.
In a typical dicing or sawing system, the fragile blade 101 is mounted on a spindle 104 as shown in FIG. 1a with a pair of flanges 103 to support the blade 101. A clearance 105 must be allowed between the flange and material to be diced 110. The clearance will change as the blade is eroded, but it must be controlled to avoid contact with the dicing subject, but yet kept as small as practical in order to avoid breaking the fragile blade. A cross-sectional view of the blade assembly is shown in FIG. 1b. 
The material to be diced 110, typically a semiconductor wafer is positioned on a piece of plastic carrier film ofter with a uv release adhesive which is secured in a supporting ring (not shown). The wafer on the tape carrier is held securely on a work surface, typically a vacuum chuck 120. Flowing water is used to cool the blade and target material, and to remove the particulate matter eroded during the sawing process.
At the semiconductor supplier, it is desirable to use the same dicing equipment not only to separate integrated circuit chips on wafers, but more recently to singulate a plurality of devices fabricated on a single circuit substrate. The substrate provides the next level of interconnection, such as a package level printed wiring circuit. Circuit substrates for integrated circuit packages are made of unfilled flexible polymeric materials such as Kapton or Upilex, of filled polymeric materials such as FR-4, FR-5 or other polymers with either fiber of particulate fillers, or of rigid, ceramic like materials. The interconnection traces are typically copper with a protective coating. Thickness of the substrates varies greatly from 0.003 inches to 0.030 inches.
The circuit substrates may further have the individual or multiple chips attached to form either an integrated circuit package, such as a Chip Scale Package (CSP) FIG. 2a, a larger similarly designed Ball Grid Array Package (BGA) or a multichip module MCM) as shown in FIG. 2b. The CSP device is generally characterized as having a package area no greater than 1.5 times that of the chip itself. A configuration, as shown in FIG. 2a, consists of a chip 210 electrically connected to a printed wiring substrate 202 by a plurality of small solder balls 211. Conductive vias (not shown) through the substrate provide contact to an array of pads, each of which has a larger solder ball 221 for making electrical contact to the next level of interconnection, typically a printed wiring board. A multichip module in FIG. 2b is similarly constructed by connecting a plurality of chips 240 to a printed circuit board substrate 242. Conductive traces (not shown) on the substrate allow connections to be made between the chips, as well as provide a means for contact to the next level of interconnection, such as solder balls 241.
Manufacturing and cost advantages of assembling a plurality of these devices as a single unit are numerous; equipment, space, labor, time and materials may all be utilized more economically and effectively by multiple, rather than single unit assembly.
However, accurately separating the substrate into individual devices having chips ranging from 0.010 to 0.050 inches in thickness presents a number of problems. As the distance between the vacuum chuck and the subject to be diced becomes larger, vibration of the high speed rotating blade may increase and add to the risk of damage to both the expensive circuits and the expensive blade. Variations in the elastic modulus of the materials to be diced contributes not only to vibration damage, but also to contamination of the saw blade with a non-abrasive, resinous material which may hinder the blade efficiency.
Unsupported structures present a particularly significant challenge to the dicing operation because they tend to tear or break rather than saw completely and cleanly.
Sawing polymeric substrates for devices such as CSP or BGA packaged integrated circuits or multichip modules presents significant challenges because the thickness of the device has increased while the dimensional precision and smoothness of the substrate edges remains unchanged. The devices require very precise control of the package dimensions and uniformity in order to insure reliable electrical contact to a test socket. Poor edge definition of the substrate can result in test yield failure at this final stage of assembly, resulting in the most costly losses. To allow dicing accuracy, the saw blades must be thin, and consequently they are somewhat fragile.
A need exists to provide a solution for precisely dicing substrates with an array of chips attached using the existing automated dicing equipment.
The principal object of the present invention is to provide a saw blade assembly for precisely separating a plurality of integrated circuit packages arrayed on a substrate. A dual saw blade assembly wherein the parallel blades are separated by a spacer and supported on the single spindle of an automated dicing system, provides a means of economically utilizing existing equipment to dice the substrate with assembled devices at very precise locations.
A dual saw blade assembly allows the use of commercially available, narrow blades, and the separation between blades is adjusted simply by selection of an inexpensive spacer or spacers inserted between the blades. Flanges are positioned on the outer surface of each blade to support the assembly, in a manner similar to the single blade assembly.
The substrate is diced from the backside in order to minimize blade exposure and to allow singulating devices much taller than the blade exposure, including devices having heat spreaders attached to the device surface.
Integrated circuit devices such as Chip Scale Packages (CSP) or Multichip Modules (MCM) fabricated on polymeric substrates require accurate sizing and precise edge acquity in order to accurately mate with contacts in test sockets. Such devices having flip chip connections are surrounded by an uneven polymeric material exuding from under the devices. This underfill material requires that the scribe streets be sufficiently wide to accommodate the out-flow, rather than abutting or closely spacing the devices. The saw blade assembly of the current invention provides a means for removing the wide streets by making two cuts simultaneously, thereby avoiding issues found when dicing unsupported structures by making two cuts with a single blade assembly. Further, the dual blade assembly decreases the process time required by making a single cut as opposed to two passes with a single blade.
The saw blade assembly is further capable of removing unwanted structures in scribe streets by selecting a spacer with width equal to or greater than the unwanted structure and the combined widths of blades and spacer is within the width of the scribe street. The blades are aligned to the street, making a single cut and enabling removal of the unwanted structures without contaminating the device from debris.